Heat and Mass Transfer Effects

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

This procedure obviously requires machine computation capability if it is to employed in reactor design calculations. Fortunately, there are many reactions for which the global rate reduces to the intrinsic rate, which avoids the necessity for calculations of this type. On the other hand, several high tonnage processes (e.g., S02 oxidation) are influenced by heat and mass transfer effects and one must be fully cognizant of their implications for design purposes. 

Self-sustained Oscillations. Under certain conditions, isothermal limit cycles in gaseous concentrations over catalysts are observed. These are probably caused by interaction of steps on the surface. Sometimes heat and mass transfer effects intervene, leading to temperature oscillations also. Since this subject has recently been reviewed (42, 43) only a few recent papers will be mentioned here. 

The N20 decomposition, CO oxidation, and H2 oxidation reactions are known to exhibit concentration oscillations over noble metal catalysts. Flytzani-Stephanopoulos et al. (47) have observed oscillations for the oxidation of NH3 over Pt. The effects are dramatic and lead to large temperature cycles for the catalyst wire. Heat and mass transfer effects are important. 

The integral reactor is useful for modeling the operations of larger packed bed units with all their heat and mass transfer effects, particularly for systems where the feed and product consist of a variety of materials. 

The design equations previously described are only valid when there are no factors which modify the kinetics of the immobilized biocatalyst (partition effects, heat and mass transfer effects and decay of biological activity) and the hydrodynamic characteristics of the reactor (back-mixing). Thus, the kinetic constants and used in those equations are intrinsic values obtained in the absence of those factors, being only dependent on the conformational and stereochemical effects inherent in the immobilization procedure used. 

The role of hydrate intrinsic kinetics has been more recently suggested to play a smaller role in hydrate growth in real systems than heat and mass transfer effects. In view of this, the discussion on the kinetics models is only briefly presented here. For a more thorough treatment, the reader is referred to the original references (Englezos et al., 1987a,b Malegaonkar et al., 1997). 

Heat and mass transfer effects can be more significant than intrinsic kinetics in multiphase systems. 

Many working groups have modeled the performance of diesel particulate traps during the past few decades. Concentrated parameter models (CSTR assumption) have been applied for the evaluation of formal kinetic models and model parameters. The formal kinetic parameters lump the heat and mass transfer effects with the reaction kinetics of the complicated reaction network of diesel soot combustion. Those models and model parameters were used for the characterization of the performance of different filter geometries and filter materials, as well as of the performance of a variety of catalytically active coatings and fuel additives [58],... 

Table 2 lists most of the available experimental criteria for intraparticle heat and mass transfer. These criteria apply to single reactions only, where it is additionally supposed that the kinetics may be described by a simple nth order power rate law. The most general of the criteria, 5 and 8 in Table 2, ensure the absence of any net effects (combined) of intraparticle temperature and concentration gradients on the observable reaction rate. However, these criteria do not guarantee that this may not be due to a compensation of heat and mass transfer effects (this point has been discussed in the previous section). In fact, this happens when y/J n [12]. 

As the true state variable of the system the rate r depends only on the temperature and concentration if it is derived from data free of heat and mass transfer effects. 

Selectivity may be determined in the integral or differential mode. Integral selectivity depends on the overall extent of the reaction (degree of conversion) and on the type of reactor used even if heat and mass transfer effects are eliminated. It may be called reactor selectivity for the formation of product P, from the set of reactants B when it is calculated as the mole fraction of P, in the products (exluding unconverted feed) at the exit of the reactor ... 

The mass transfer effects cause, in general, a decrease of the measured reaction rate. The heat transfer effects may lead in the case of endothermic reactions also to a decrease of the equilibrium value and the resulting negative effect may be more pronounced. With exothermic reactions, an insufficient heat removal causes an increase of the reaction rate. In such a case, if both the heat and mass transfer effects are operating, they can either compensate each other or one of them prevails. In the case of internal transfer, mass transport effects are usually more important than heat transport, but in the case of external transfer the opposite prevails. Heat transport effects frequently play a more important role, especially in catalytic reactions of gases. The influence of heat and mass transfer effects should be evaluated before the determination of kinetics. These effects should preferably be completely eliminated. 

Additionally, the rate of heat transfer may also become important. Nonuniform temperature distributions within the solid particles result in differing local rates of reaction, as the reaction rates are strongly depending on the temperature according to the Arrhenius law. Heat- and mass-transfer effects become increasingly important with increasing rates of reaction [1]. Whereas the macroscopic kinetics describe the rate of a chemical reaction, thermodynamics determines the maximum extent to which reactions can occur. Provided that the rate of reaction is sufficiently fast, the thermodynamical equilibrium can be reached. 

Internal recycle reactors are designed so that the relative velocity between the catalyst and the fluid phase is increased without increasing the overall feed and outlet flow rates. This facilitates the interphase heat and mass transfer rates. A typical internal flow recycle stirred reactor design proposed by Berty (1974, 1979) is shown in Fig. 18. This type of reactor is ideally suited for laboratory kinetic studies. The reactor, however, works better at higher pressure than at lower pressure. The other types of internal recycle reactors that can be effectively used for gas-liquid-solid reactions are those with a fixed bed of catalyst in a basket placed at the wall or at the center. Brown (1969) showed that imperfect mixing and heat and mass transfer effects are absent above a stirrer speed of about 2,000 rpm. Some important features of internal recycle reactors are listed in Table XII. The information on gas-liquid and liquid-solid mass transfer coefficients in these reactors is rather limited, and more work in this area is necessary. 

Sampling and analysis of product composition are good - only normal problems are encountered. The reactor is well mixed and isothermal conditions in the reactor can be maintained. Due to well-mixed conditions, the extraneous heat- and mass-transfer effects are at a minimum. 

Similar tests of the fiuidized-bed method have been successful with a variety of molecular adsorbates and catalysts (other zeolites, supported oxides, naphthalene, pyridine, methanol, alkanes, alkenes, acetonitrile, ammonia, etc.) (25). We believe that this fiuidized-bed method is a major step forward for measurements of working catalysts with UV Raman spectroscopy. It should also be a useful method for measurements of catalytic kinetics by reducing heat and mass transfer effects that arise when catalysts are used in the form of pellets. In the limit of low conversions... 

Hysteresis loops were observed both in the low and high activity states. The positions of the ramps up and down half-cycles of the hysteresis loop as well as its width strongly depended on the experimental conditions as we already discussed and published [18, 22]. The similar behaviour of the catalysts in the combustion of m-xylene in the low and high activity states points to the absence of heat and mass transfer effect. 

Under these circumstances, the interparticle transport resistances can be neglected. What are left are the intraparticle resistances, i.e. the heat and mass transfer effects inside the catalyst particles. Since the current case reflects the situation that few reactant and product molecules exist in an environment of solvent molecules, the simplest Fick s law approach with effective diffusion coefficients can be considered as sufficient for the description of molecular diffusion. 

Hanika J., K. Sporka, V. Ruzicka and J. Krausova, Qualitative Observations of Heat and Mass Transfer Effects on the Behaviour of a Trickle Bed Reactor , Chem Eng Comm., 2, 19 (1975). 

The estimated model parameters are given in table 1. Note that the estimated model parameters cannot be considered to represent intrinsic kinetic constants. They represent lumped parameters which can be disguised by possible heat and mass transfer effects which are not accounted for in the model. 

The figures reveal a hot spot in the bed, which is typical for strongly exothermic processes. The magnitude of this hot spot depends, of course, on the heat effect of the reaction, the rate of reaction, the heat transfer coefficient and transfer areas as shown by Bilous and Amundson [21]. Its location depends on the flow velocity. It is also observed that the profiles become sensitive to the parameters from certain values onward. If the partial pressure of the hydrocarbon were 0.018 atm an increase of 0.0002 atm woifld raise the hot spot temperature beyond permissible limits. Such a phenomenon is called runaway. Note that for the upper part of the curves with po = 0.0181, 0.0182, and 0.019 (Figs. 11.5.b-l and 2) the model used here is not longer entirely adequate heat and mass transfer effects would have to be taken into account There is no doubt however as to the validity of the lower part indicating excessive sensitivity in this region. 

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